Make Database @Transactions a compare by identity hash

This is faster, and would fix a theoretical bug in the case where
a driver implemented a connection object that could compare equal
(hash/eql?) with another connection object.

lib/sequel/database/misc.rb

@@ -149,6 +149,7 @@ def initialize(opts = OPTS)
       @schemas = {}
       @prepared_statements = {}
       @transactions = {}
+      @transactions.compare_by_identity
       @symbol_literal_cache = {}
 
       @timezone = nil